An adult Belgian patient with visceral leishmaniasis acquired while on holiday in Spain is described. This patient's cellular immunity was assessed by skin testing using six different antigens and was normal just before leaving for Spain. During the leishmaniasis, skin tests with the same antigens showed complete anergy. Lymphocyte transformation tests to antigens and mitogens were severely impaired but normal T cell subsets determined by OKT monoclonal antibodies were found. Immune complexes, containing antibodies to Leishmania antigen were isolated during the patient's illness. It is most likely that visceral leishmaniasis caused the functional cellular immune impairment of the patient.